Resumen de Efectos y estructuras basadas en la dispersión de brillouin para nuevos dispositivos y sensores de fibra óptica
This thesis is based on the interaction between electromagnetic waves and sound waves characteristic of the structure of the material through which light propagates. This interaction is known as Brillouin scattering that is a nonlinear effect of great importance in the field of fiber optics. The purpose of this thesis work is to contribute to the advancement of the fiber Brillouin technique. These contributions are arranged in four parts, the study of the chirp factor in distributed Brillouin sensors, the influence of the shape of the pulses in the performance of Brillouin sensors, the Brillouin frequency in optical fibers and its dependence on the humidity and finally optical devices based on the tailoring of Brillouin scattering.
The influence of the chirp factor on the accurate measurement of Brillouin frequency is demonstrated. Theoretical simulations and experimental tests using standard intensity modulators with a real chirp factor have shown non-negligible Brillouin frequency deviations. This deviation under some circumstances exceeded the specific accuracy of distributed sensors and it is particularly important when the pulse shows a pronounced asymmetry and an overshoot on the leading edge (or trailing edge). Moreover, the impact of the chirp on the Brillouin gain spectra in the transition regions (regions of change in the Brillouin frequency) by using short pump pulse is experimentally shown. From this research, some suggestions are pointed out for minimizing the chirp biasing when designing the setup: As far as possible, selecting a modulator with a reduced chirp parameter making the pulse with sharp rising and falling edges although it provokes a broadening in the pulse spectrum and finally, paying attention to suppress overshoots in the electrical driving pulse since those overshoots are highly detrimental in the optical signal quality.
Furthermore, the efficient cancellation of chirp by using a sequence of electrical pulses -the pulses applied in the RF connector of the EOM- with equal amplitude but reverse polarity was proposed.
In order to improve the spatial resolution in a distributed Brillouin sensor, the pump pulse shape is investigated. A set of non trivial pulse configurations is proposed and the results are compared with the data corresponding to a typical rectangular pulse. In real experiments, short pulses did not look like perfect rectangles, hence it is important to determine how they change and affect the resultant Brillouin spectrum after the convolution process between the original g(v) and the spectral distribution of the pulse f(v). Pulses with shapes such as saw tooth, inverted saw tooth, triangular, sinc and rectangular are simulated and measured. A valuable result of the experiments is that, it is possible to use other pulse shapes like the saw tooth to get similar results using the Brillouin scattering. Besides, it can be pointed out that the main rectangular pulse can be accompanied of other close pulses, which serve as a seed or as a way to preserve the Stokes signal.
The effects of water vapor concentrations on the measurement of Brillouin frequency shift while the vapor concentration in the environmental media is varied are experimentally investigated. It is important to mention that the reported pattern in Brillouin frequency shift as a function of the relative humidity is also observed in fiber Bragg grating. Here, the shift due to the thermo optic effect fits with the Brillouin frequency shift for relative humidity changes at a constant temperature. The Brillouin frequency shift depends on the water vapor concentration in the air and the temperature. At a high RH, the absolute humidity increases and the fiber reaches more efficiently the set temperature, hence the variation in Brillouin shift depends on the effective heat transfer between the air-silica or air-jacket-silica media. The measurement accuracy of the environmental temperature has to include the uncertainty caused by the humidity in a Brillouin temperature sensor.
Optical active devices based on the handling of the Brillouin spectral response are presented. These optical devices can be made by reshaping the Brillouin gain spectrum in optical fibers. Furthermore, by using separately or combined the techniques of serial and parallel fiber concatenation, the Brillouin frequency distribution of these devices can be tailored. Besides, the Brillouin dependence on temperature and strain can be useful as a tuning factor of the BFS. The sensing and the Brillouin frequency measurement by exploiting the Brillouin amplification obtained in a fiber ring cavity are proposed and experimentally validated. As an example of a sensor configuration based on Brillouin ring lasing, a simple system for measuring temperature in multiple zones in a quasi distributed manner is shown. Optical fiber reels are serially concatenated and separated by zones. Each spool of fiber has a characteristic Brillouin wavelength that depends on the fiber properties and the temperature in the zone. The different temperatures are independently and accurately measured through heterodyne detection between two narrow laser signals. This method avoids the usage of electro-optic modulators and allows the usage of strong signals to determine the frequency variations. This sensor configuration may determine the integral temperature of a fiber in real time. Also, the power intensity threshold can be set to improve the signal amplitude by choosing diverse fiber lengths. Finally, the Brillouin ring amplification is also used for measuring the dual Brillouin frequency peak present in the spectral distribution of a photonic crystal fiber. The ring configuration increases the efficiency of the backscattered signal and helps to improve the Brillouin detection and characterization.
